As early as 1988, scientists cautioned that human tinkering with the Earth's climate amounted to "an unintended, uncontrolled globally pervasive experiment whose ultimate consequences could be second only to a global nuclear war." Since then, hundreds of scientific studies have documented ever-mounting evidence that human activities are altering the climate around the world. A growing number of international leaders now warn that climate change is, in the words of U.K. Chief Scientific Advisor David King, "the most severe problem that we are facing today—more serious even than the threat of terrorism."

Climate change will likely trigger severe disruptions with ever-widening consequences for local, regional, and global security. Droughts, famines, and weather-related disasters could claim thousands or even millions of lives and exacerbate existing tensions within and among nations, fomenting diplomatic and trade disputes. In the worst case, further warming will reduce the capacities of Earth's natural systems and elevate already-rising sea levels, which could threaten the very survival of low-lying island nations, destabilize the global economy and geopolitical balance, and incite violent conflict.

Already, there is growing evidence that climate change is affecting the life-support systems on which humans and other species depend. And these impacts are arriving faster than many climate scientists predicted. Recent studies have revealed changes in the breeding and migratory patterns of animals worldwide, from sea turtles to polar bears. Mountain glaciers are shrinking at ever-faster rates, threatening water supplies for millions of people and plant and animal species. Average global sea level has risen 20-25 centimeters (8-10 inches) since 1901, due mainly to thermal expansion; more than 2.5 centimeters (one inch) of this rise occurred over the past decade.

A recent report by the International Climate Change Taskforce, co-chaired by Republican U.S. Senator Olympia Snowe, concludes that climate change is the "single most important long term issue that the planet faces." It warns that if average global temperatures increase more than two degrees Celsius—which will likely occur in a matter of decades if we continue with business-as-usual—the world will reach the "point of no return," where societies may be unable to cope with the accelerating rates of change.

Existing threats to security will be amplified as climate change has increasing impacts on regional water supplies, agricultural productivity, human and ecosystem health, infrastructure, financial flows and economies, and patterns of international migration. Specific threats to human welfare and global security include:

► Climate change will undermine efforts to mitigate world poverty, directly threatening people's homes and livelihoods through increased storms, droughts, disease, and other stressors. Not only could this impede development, it might also increase national and regional instability and intensify income disparities between rich and poor. This, in turn, could lead to military confrontations over distribution of the world's wealth, or could feed terrorism or transnational crime.

► Rising temperatures, droughts, and floods, and the increasing acidity of ocean waters, coupled with an expanding human population, could further stress an already limited global food supply, dramatically increasing food prices and potentially triggering internal unrest or the use of food as a weapon. Even the modest warming experienced to date has affected fisheries and agricultural productivity, with a 10 percent decrease in corn yields across the U.S. Midwest seen per degree of warming.

► Altered rainfall patterns could heighten tensions over the use of shared water bodies and increase the likelihood of violent conflict over water resources. It is estimated that about 1.4 billion people already live in areas that are water-stressed. Up to 5 billion people (most of the world's current population) could be living in such regions by 2025.

► Widespread impacts of climate change could lead to waves of migration, threatening international stability. One study estimates that by 2050, as many as 150 million people may have fled coastlines vulnerable to rising sea levels, storms or floods, or agricultural land too arid to cultivate. Historically, migration to urban areas has stressed limited services and infrastructure, inciting crime or insurgency movements, while migration across borders has frequently led to violent clashes over land and resources.

The parallels with terrorism are compelling. Traditional responses to security threats cannot address the root of such problems, and related impacts could persist even if global emissions are cut dramatically over coming decades because of the significant lag time between cause and effect. As with terrorism, we know that changes will occur, but not when or where they will strike, nor how damaging and costly they will be. Climate change already claims more lives than does terrorism: according to the World Health Organization, global climate change now accounts for more than 160,000 deaths annually. By the time the world experiences the climate equivalent of September 11th, or the 2004 Madrid bombings, it could be too late to respond.

Fortunately, we already have the means to address the problem. In order to prevent dangerous climate change, we will need to invest in preventive measures—just as the U.S. is spending heavily on homeland security to reduce the risk of terrorism. The Kyoto Protocol is a first step in a comprehensive security program. That measure, which binds its signatories to reduce their emissions, entered into force this February—but without the world's largest greenhouse gas emitter, the United States. The Kyoto Protocol is a small but crucial step in the direction of a more stable climate; the next step is to form a broad global consensus around the need to shift to a clean, efficient energy system based increasingly on renewable energy sources like wind and solar power. And to be effective, the consensus must include the world's largest emitters—including the United States and China.

The global market for renewable energy exceeded $25 billion in 2004, and these technologies are already reducing emissions in countries such as Germany and Spain, while providing new jobs and significant business and economic opportunities. Dramatically expanding investments in renewable energy and efficiency will ensure not only a cleaner, healthier future, but a more secure world as well.

Burgeoning markets for clean energy technologies such as solar panels, wind turbines, and biofuels, and rising demand for "green buildings" and fuel-efficient vehicles show what is possible when political will and the right policies unite to ensure that energy markets work efficiently.

Policy initiatives that would speed the development of a more energy-efficient and climate-friendly global economy include:

Enact standards and market incentives that promote more-efficient buildings, vehicles, and appliances to dramatically increase energy conservation and efficiency. Despite major advances since the first oil crisis, significant potential for improvement remains in both industrial and developing nations.

Set aggressive targets and provide market incentives for the rapid and large-scale introduction of renewable energy technologies for electricity generation, heating and cooling, and transportation.

Internalize the full costs—environmental, health, and security—of energy production and use, and replace subsidies for mature, polluting fuels and technologies with targeted incentives for renewable technologies that sunset as they become competitive.

About the author: Janet Sawin is a senior researcher at the Worldwatch Institute and director of the Institute's Energy and Climate Change Program. She is co-author of the chapter "Changing the Oil Economy" in State of the World 2005.